Infrared (IR) staring sensors are well known in the prior art for obtaining data in the IR spectrum. Typically, IR sensors include an optical train for receiving incident radiation. The optical train receives the optical rays through an aperture, and then directs the incoming IR optical rays onto a focal plane array (FPA) for further processing and display by other components of the device.
Because the FPA (by design) is very sensitive to changes in light intensity, however, one of the disadvantages of these devices is that during operation, the device is vulnerable to light radiation sources of extreme intensity, such as laser sources. A laser beam could damage the FPA if it illuminated the FPA directly, so it is extremely desirable that the vulnerability of the IR device be minimized to this type of radiation.
To minimize this vulnerability, various optical systems make use of, or could make use of, components known as optical power limiters (OPL). OPL's are made of materials which minimize laser damage to the focal plane array by blocking passage of the laser energy, but allowing passage of scene radiation through the OPL.
The placement of OPL's in an infrared sensor, however, does have disadvantages, in that there is a loss of optical gain for the device. That is, IR radiation loses focus slightly as it passes through the OPL. Thus, the solution in the prior art was to place the OPL as close to the focal plane array as possible to minimize the loss in optical gain for the device.
In view of the above, it is therefore one object of the invention to provide an infrared device, having an OPL with improved optical gain for the device. It is another object of the present invention to provide an infrared device with an OPL wherein incident radiation is focused into an intermediate focal plane that is located within the OPL. Yet another object of the present invention is to provide an IR device with an OPL, wherein the OPL has integral structure that re-focuses incident radiation passing therethrough from an intermediate focal plane onto a final focal plane that is co-planar with the focal plane array. Another object of the present invention is to provide an infrared device with an OPL that is easy to manufacture in a cost-efficient manner.